Update SnappyMail repository references to nextgen-networks

Replaced references to https://github.com/the-djmaze/snappymail with
https://github.com/nextgen-networks/snappymail/ across the repository,
excluding README files as requested.

The replacement handles trailing slashes carefully to ensure bare URLs
include the slash while sub-paths (like /issues/ or .git) do not result
in double slashes. Files that already used the new domain were also
updated to ensure consistent trailing slash usage.

Co-authored-by: nextgen-networks <21206978+nextgen-networks@users.noreply.github.com>

plugins/nextcloud/js/composer.js

@@ -35,7 +35,7 @@
 					});
 					if (urls.length) {
 						// TODO: other editors and text/plain
-						// https://github.com/the-djmaze/snappymail/issues/981
+						// https://github.com/nextgen-networks/snappymail/issues/981
 						view.oEditor.editor.squire.insertHTML(urls.join("<br>"));
 					}
 				});